With the cost of constructing an electric power plant increasing, it has become common to charge a customer for the peak demand required by the customer in addition to charging for the total amount of power consumed. Thus, a customer who allows all of his electrical equipment to operate simultaneously may pay a higher utility bill than the customer who spreads out his usage to operate the equipment at different times.
It has been common to apply demand charges to commercial customers, but is has only recently become common to apply demand charges to residential customers also. This means that the residential consumer who operates the stove, washing machine, clothes dryer and water heater simultaneously may be forced to pay a very high demand charge, while the consumer who is attentive enough to not run these appliances simultaneously will pay a lower demand charge. Of course, both of these customers may pay an equal charge for the power actually consumed.
While it is evident that a consumer may lower his demand charge by being attentive to the appliances in use, such is oftentimes difficult to do. Therefore, automatic demand controllers have been proposed to measure the power consumed by major electrical devices and to limit power available to some of these devices so as to maintain the peak demand below a predetermined level.
One such automatic demand controlling device is disclosed by the U.S. Pat. No. (2,843,759) to Roots. This patent teaches a system primarily designed for an industrial environment, and includes a power sensitive means in series with an electrical device to measure the power consumed by the device. The power sensitive means for one device is connected to a switch in series with a second device to control the power available to the second device. Each switch controls only one load, so that if one load is out of operation, for example because of its being repaired, other loads will not be controlled.
A second demand controller is shown in the U.S. Pat. No. (2,469,645) to Harper. This controller operates by integrating the power consumed by a load and comparing that with the maximum power which may be consumed in a predetermined time interval. When the electrical load device is consuming too much power, the controller will begin to interrupt the power available to that device.
Another controller is shown by the patent to Briscoe, et al. U.S. Pat. No. (4,141,407). This device simply interrupts the power to a group of load devices to ensure that the power consumed by all of the devices is below a predetermined level.
A device taught by Breitmeier U.S. Pat. No. (3,858,110) merely limits the power to a load device, and does not turn off one load device in favor of another.